Process and apparatus for production of ammonia synthesis gas and pure methane by cryogenic separation

ABSTRACT

A process for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas containing at least 75% hydrogen as well as methane and nitrogen, the feed gas is cooled in a heat exchanger and partially condensed, the partially condensed feed is sent to a phase separator, the gas from the phase separator is sent to a nitrogen wash column, liquid nitrogen is sent to the top of the nitrogen wash column and ammonia synthesis gas is removed from the top of the nitrogen wash column, the liquid from the phase separator is expanded and sent to a separation column, a nitrogen enriched gas stream is removed from the top of the column and a nitrogen depleted liquid stream rich in methane is removed from the bottom of the column and the nitrogen depleted liquid stream rich in methane is vaporized to form a pure methane product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/CN2011/070315, filed Jan. 17, 2011, which is herein incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process and apparatus for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas.

BACKGROUND

Ammonia synthesis plants are fed by a mixture of hydrogen and nitrogen in stoichiometric proportions. Certain components, such as argon, methane, oxygen, carbon monoxide or carbon dioxide are considered as inert or catalyst poisons for the synthesis process.

Thus the hydrogen which is to be mixed with the nitrogen needs to be purified. This hydrogen may be produced by a steam reformer or a coal gasification process for example.

It is known from DE-A-2814660 to produce the ammonia synthesis gas by washing the hydrogen in a nitrogen wash process in which liquid nitrogen is sent to the top of the wash column.

In this case, an off gas from an acetylene plant is treated by partial condensation to produce a hydrogen stream and the hydrogen stream is sent to a nitrogen wash column. The liquid from the partial condensation is further treated in a column.

SUMMARY OF THE INVENTION

One object of the present invention is to treat the liquid from the partial condensation to produce a methane rich stream. In particular, it is desired to produce a methane riche stream containing very little nitrogen. If the methane is subsequently used to generate synthesis gas, the presence of nitrogen in the methane increases the size of the heaters and the unit used to form the gas.

In addition, if the synthesis gas formed using the nitrogen containing methane is in its turn used to feed a methanol synthesis unit, the presence of nitrogen reduces the product because it increases the size of the purge stream

It is consequently desirable to reduce the nitrogen content of the stream.

According to an object of the invention, there is provided a process for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas containing at least 75% hydrogen as well as methane and nitrogen, preferably at least 5% nitrogen wherein the feed gas is cooled in a heat exchanger and partially condensed, the partially condensed feed is sent to a phase separator, the gas from the phase separator is sent to a nitrogen wash column, liquid nitrogen is sent to the top of the nitrogen wash column and ammonia synthesis gas is removed from the top of the nitrogen wash column, the liquid from the phase separator is expanded and sent to a separation column, a nitrogen enriched gas stream is removed from the top of the column and a nitrogen depleted liquid stream rich in methane is removed from the bottom of the column and the nitrogen depleted liquid stream rich in methane is vaporised to form a pure methane product.

According to further optional embodiments:

-   -   the nitrogen enriched gas stream contains at least 50% nitrogen         and/or at least 20% carbon monoxide.     -   the nitrogen depleted stream contains at least 80% methane,         preferably at least 85% methane, and at most 2% nitrogen,         preferably at most 1.5% nitrogen.     -   the gas from the phase separator contains at least 90% hydrogen.     -   the nitrogen wash column operates at between 25 and 35 bars.     -   the separation column operates at between 1.5 and 8 bars.     -   the separation column has a bottom reboiler and at least a part         of the feed gas and/or a gas derived from the feed gas is used         to heat the bottom reboiler.     -   feed gas is cooled and partially condensed to form a gas and a         liquid and at least a part of the gas resulting from the partial         condensation is used to reboil the separation column.     -   liquid from the bottom of the nitrogen wash column is sent to         the top of the separation column.     -   the separation column has a top condenser and wherein liquid         nitrogen is vaporised in the top condenser, the nitrogen         enriched gas stream is partially condensed in the top condenser         and the liquid from the phase separator is sent to an         intermediate region of the separation column.     -   the separation column has no top condenser and wherein liquid         from the phase separator is sent to the top of the separation         column.     -   liquid from the bottom of the nitrogen wash column is vaporised         in the heat exchanger.     -   the liquid sent to the separation column from the phase         separator contains at least 65% methane, preferably at least 70%         methane and at least 5% nitrogen, preferably at least 10%         nitrogen.

According to a further object of the invention, there is provided an apparatus for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas containing at least 75% hydrogen as well as methane and nitrogen, preferably at least 5% nitrogen comprising a heat exchanger wherein the feed gas is cooled and partially condensed, a phase separator, a conduit for sending the partially condensed feed to the phase separator, a nitrogen wash column, a separation column, a conduit for sending gas from the phase separator to the nitrogen wash column, a conduit for sending liquid nitrogen to the top of the nitrogen wash column, a conduit for removing ammonia synthesis gas from the top of the nitrogen wash column, a valve for expanding liquid from the phase separator connected to the separation column via a conduit, a conduit for removing a nitrogen enriched gas stream from the top of the column and a conduit for removing nitrogen depleted liquid stream rich in methane from the bottom of the column and means for vaporising the nitrogen depleted liquid stream rich in methane to form a pure methane product.

According to further optional features:

-   -   the separation column has a bottom reboiler and comprising a         conduit for sending at least a part of the feed gas and/or a gas         derived from the feed gas to heat the bottom reboiler.     -   the apparatus comprises means for cooling and partially         condensing the feed gas to form a gas and a liquid and means for         sending at least a part of the gas resulting from the partial         condensation to the reboiler.     -   the apparatus comprises a conduit for sending liquid from the         bottom of the nitrogen wash column to the top of the separation         column.     -   the separation column has a top condenser and the apparatus         comprises a conduit for sending liquid nitrogen to the top         condenser, a conduit for sending the nitrogen enriched gas         stream to the top condenser and a conduit for sending the liquid         from the phase separator to an intermediate region of the         separation column.     -   the separation column has no top condenser and the apparatus         comprises a conduit for sending liquid from the phase separator         to the top of the separation column.     -   a conduit for sending liquid from the bottom of the nitrogen         wash column to be vaporised in the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 is a process in accordance with an embodiment of the invention.

FIG. 2 is a process in accordance with an embodiment of the invention.

FIG. 3 is a process in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

The feed gas preferably contains at least 75%.hydrogen and at least 5%.methane.

The term “pure methane” means that the methane produced contains at least 80% methane, preferably at least 85% methane and preferably less than 2% nitrogen, or even less than 1.5% nitrogen.

All the purities in this document are expressed in molar percentages. All pressures are absolute pressures.

The invention will be described in more detail, referring to FIGS. 1 to 3 which represent processes according to the invention.

In FIG. 1, a hydrogen rich gas derived from a Rectisol Unit® has previously been purified in a molecular sieve purification unit to remove carbon dioxide and methanol.

This feed gas 1 at between 30 and 40 bars and at −53° C. contains at least 75% hydrogen, at least 2% nitrogen and at least 6% methane and may also contain carbon monoxide, argon or hydrocarbon impurities, such as ethane. The feed gas 1 is divided in two parts 5, 3. One part 5 is cooled in the main exchanger line 7 and partially condensed before being sent to first phase separator 9. The main heat exchanger line is in two sections 7, 85, section 7 being the colder of the two.

The vapour phase 6 from the phase separator 9 is mixed with the other part 3 of the feed gas to form gas stream 11 The gas stream 11 is sent to bottom reboiler 21 of the separation column 19 and then to a second phase separator 23. The gas 25 from the second phase separator 23 is sent to the main heat exchanger line 7 to be cooled and then to the third phase separator 31. The gas 25 may be mixed with part 81 of feed gas 11 after expansion in valve 81. The top gas from the third separator 31 is sent to the bottom of nitrogen washing column 45 as feed. The top gas 33 contains 93% H₂; 4% N₂; 2% CO and 1% CH₄.

The liquid 35 from the third phase separator 31 is sent to separation column 19 after expansion in valve 37. The liquid 35 contains at least 65% methane or even at least 70% methane with at least 5% nitrogen or even at least 10% nitrogen, the balance being made up of hydrogen, carbon monoxide and other impurities. The bottom liquid 39 of the column contains at least 80% methane, preferably at least 85% methane with less than 2% of N₂, still more preferably with less than 1.5% nitrogen. The CH₄ rich liquid is expanded in valve 41 and sent to fourth phase separator 17. The liquid 73 from the fourth phase separator 17 after vaporization and warming up in the main heat exchanger line 7 is sent to battery limit to be used for example as feed to steam reformer after compression. The gas 71 from the fourth phase separator 17 is also sent to the battery limit after warming in the main heat exchange line 7.

Top gas 43 from the separation column 19 contains at least 50% nitrogen, at least 20% carbon monoxide and a little hydrogen and methane and is used as fuel after being warmed up in the main exchanger line 7. The separation column 19 has no top condenser.

Reboiling duty of separation column 19 is provided by cooling down of the feed gas 1 and/or a gas derived from the feed gas 1 by cooling and phase separation.

Liquid 13 from the first phase separator 9 is expanded in valve 15 and sent to the fourth phase separator 17.

Liquid 27 from the second phase separator 23 is expanded in valve 29 and sent to the fourth phase separator 17.

The high pressure nitrogen gas HPN₂ 65 at around 40 bars is cooled down and liquefied in the main exchanger line 7. Part of the high pressure nitrogen HPN₂ is sent to the top of the column 45 and the rest of the HPN₂ part 59 is mixed with H₂ rich gas 57 coming from nitrogen wash column 45 according to the requirements of the ammonia synthesis.

One part 69 of the product forming ammonia synthesis gas from the top of the nitrogen washing column 45 is warmed up to ambient temperature through the main exchanger line 85. Another part 67 of the ammonia synthesis gas product will be warmed up in the Rectisol® exchanger for cold recovery.

The ammonia synthesis gas 67, 69 contains 75% hydrogen and 25% nitrogen.

The nitrogen wash column bottom liquid 47 is expanded in valve 49 to low pressure (for example 3-5 bar) and sent to a fifth phase separator 51. The gas 53 and the liquid 55 are sent to the main exchanger line 7 separately for cold recovery. These two fluids 53, 55 are mixed after vaporization of the liquid and sent to the battery limits as the fuel gas.

The cold balance is achieved by injection of liquid nitrogen supplied at battery limit from an air separation unit.

The nitrogen wash column 45 operates at between 25 and 35 bars and the separation column 19 at between 1, 5 and 7 bars.

In FIG. 2, a different separation column is used, having a top condenser 75 in addition to bottom reboiler 21. The fifth phase separator 51 is eliminated.

The hydrogen rich gas derived from a Rectisol Unit® has previously been purified in a molecular sieve purification unit to remove carbon dioxide and methanol.

This feed gas 1 at between 30 and 40 bars and at −53° C. contains at least 75% hydrogen, at least 2% nitrogen and at least 6% methane and may also contain carbon monoxide, argon or hydrocarbon impurities, such as ethane. The feed gas 1 is divided in two parts 5, 3. One part 5 is cooled in the main exchanger line 7 and partially condensed before being sent to first phase separator 9.

The vapour phase 6 from the phase separator 9 is mixed with the other part 3 of the feed gas to form gas stream 11 The gas stream 11 is sent to bottom reboiler 21 of the separation column 19 and then to a second phase separator 23. The gas 25 from the second phase separator 23 is sent to the main heat exchanger line 7 to be cooled and then to the third phase separator 31. The gas 25 may be mixed with part 81 of feed gas 11 after expansion in valve 81. The top gas from the third separator 31 is sent to the bottom of nitrogen washing column 45 as feed. The top gas 33 contains 93% H₂; 4% N₂; 2% CO and 1% CH₄.

The liquid 35 from the third phase separator 31 is sent to the middle of the separation column 19 after expansion in valve 37. The liquid 35 contains at least 65% methane or even at least 70% methane with at least 5% nitrogen or even at least 10% nitrogen, the balance being made up of hydrogen, carbon monoxide and other impurities. The bottom liquid 39 of the column contains at least 80% methane, preferably at least 85% methane with less than 2% of N₂, still more preferably with less than 1.5% nitrogen. The CH₄ rich liquid is expanded in valve 41 and sent to fourth phase separator 17. The liquid 73 from the fourth phase separator 17 after vaporization and warming up in the main heat exchanger line 7 is sent to battery limit to be used for example as feed to steam reformer after compression. The gas 71 from the fourth phase separator 17 is also sent to the battery limit after warming in the main heat exchange line 7.

Top gas 43 from the separation column 19 contains at least 50% nitrogen, at least 20% carbon monoxide and a little hydrogen and methane and is sent to top condenser 75 to be condensed by heat exchange with liquid nitrogen LIN. Part 43 of the top gas is used as fuel after being warmed up in the main exchanger line 7.

Reboiling duty of separation column 19 is provided by cooling down of the feed gas 1 and/or a gas derived from the feed gas 1 by cooling and phase separation.

Liquid 13 from the first phase separator 9 is expanded in valve 15 and sent to the fourth phase separator 17.

Liquid 27 from the second phase separator 23 is expanded in valve 29 and sent to the fourth phase separator 17.

The high pressure nitrogen gas HPN₂ 65 at around 40 bars is cooled down and liquefied in the main exchanger line 7. Part of the high pressure nitrogen HPN₂ is sent to the top of the column 45 and the rest of the HPN₂ part 59 is mixed with H₂ rich gas 57 coming from nitrogen wash column 45 according to the requirements of the ammonia synthesis.

One part 69 of the product forming ammonia synthesis gas from the top of the nitrogen washing column 45 is warmed up to ambient temperature through the main exchanger line 7. Another part 67 of the ammonia synthesis gas product will be warmed up in the Rectisol® exchanger for cold recovery.

The ammonia synthesis gas 67, 69 contains 75% hydrogen and 25% nitrogen.

The nitrogen wash column bottom liquid is expanded in valve 49 to low pressure (for example 1.5-7 bar) and sent to the top of the separation column 19 in liquid form. Vaporised nitrogen 77 from the condenser 75 is warmed in the main heat exchanger 7.

The cold balance is achieved by injection of liquid nitrogen supplied at battery limit from an air separation unit.

In FIG. 3 a variant of FIG. 1 is shown with a sixth phase separator in series with the fifth phase separator. As before a hydrogen rich gas derived from a Rectisol Unit® has previously been purified in a molecular sieve purification unit to remove carbon dioxide and methanol.

This feed gas 1 at between 30 and 40 bars and at −53° C. contains at least 75% hydrogen, at least 2% nitrogen and at least 6% methane and may also contain carbon monoxide, argon or hydrocarbon impurities, such as ethane. The feed gas 1 is divided in two parts 5, 3. One part 5 is cooled in the main exchanger line 7 and partially condensed before being sent to first phase separator 9.

The vapour phase 6 from the phase separator 9 is mixed with the other part 3 of the feed gas to form gas stream 11 The gas stream 11 is sent to bottom reboiler 21 of the separation column 19 and then to a second phase separator 23. The gas 25 from the second phase separator 23 is sent to the main heat exchanger line 7 to be cooled and then to the third phase separator 31. The gas 25 may be mixed with part 81 of feed gas 11 after expansion in valve 81. The top gas from the third separator 31 is sent to the bottom of nitrogen washing column 45 as feed. The top gas 33 contains 93% H₂; 4% N₂; 2% CO and 1% CH₄.

The liquid 35 from the third phase separator 31 is sent to separation column 19 after expansion in valve 37. The liquid 35 contains at least 65% methane or at least 70% methane with at least 5% nitrogen or even at least 10% nitrogen, the balance being made up of hydrogen, carbon monoxide and other impurities. The bottom liquid 39 of the column contains at least 80% methane, preferably at least 85% methane with less than 2% of N2, still more preferably with less than 1.5% nitrogen. The CH₄ rich liquid is expanded in valve 41 and sent to fourth phase separator 17. The liquid 73 from the fourth phase separator 17 after vaporization and warming up in the main heat exchanger line 7 is sent to battery limit to be used for example as feed to steam reformer after compression. The gas 71 from the fourth phase separator 17 is also sent to the battery limit after warming in the main heat exchange line 7.

Top gas 43 from the separation column 19 contains at least 50% nitrogen, at least 20% carbon monoxide and a little hydrogen and methane and is used as fuel after being warmed up in the main exchanger line 7. The separation column 19 has no top condenser.

Reboiling duty of separation column 19 is provided by cooling down of the feed gas 1 and/or a gas derived from the feed gas 1 by cooling and phase separation.

Liquid 13 from the first phase separator 9 is expanded in valve 15 and sent to the fourth phase separator 17.

Liquid 27 from the second phase separator 23 is expanded in valve 29 and sent to the fourth phase separator 17.

The high pressure nitrogen gas HPN₂ 65 at around 40 bars is cooled down and liquefied in the main exchanger line 7. Part of High pressure nitrogen HPN₂ is sent to the top of the column 45 and the rest of the HPN₂ part 59 is mixed with H₂ rich gas 57 coming from nitrogen wash column 45 according to the requirements of the ammonia synthesis.

One part 69 of the product forming ammonia synthesis gas from the top of the nitrogen washing column 45 is warmed up to ambient temperature through the main exchanger line 7. Another part 67 of the ammonia synthesis gas product will be warmed up in the Rectisol® exchanger for cold recovery.

The ammonia synthesis gas 67, 69 contains 75% hydrogen and 25% nitrogen.

The nitrogen wash column bottom liquid 47 is expanded in valve 49 to medium pressure (for example 7-14 bar) and sent to a fifth phase separator 51. The gas 53, rich in hydrogen, is sent to the main exchanger line 7 for cold recovery. Gas 53 can be sent to the inlet of the Rectisol® recycle compressor to increase the hydrogen yield.

The liquid from the fifth phase separator 51 is expanded in valve 89 and then sent to sixth phase separator 91. The gas 93 from the sixth phase separator 91 is warmed in heat exchangers 7, 85 and the liquid 95 is vaporized in heat exchange line 7 and warmed in heat exchanger 85.

The cold balance is achieved by injection of liquid nitrogen supplied at battery limit from an air separation unit.

For all the figures, the nitrogen wash column 45 operates at between 25 and 35 bars and the separation column 19 at between 1, 5 and 7 bars.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. A process for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas containing at least 75% hydrogen as well as methane and nitrogen, the process comprising the steps of: cooling and partially condensing the feed gas in a heat exchanger to form a partially condensed feed; introducing the partially condensed feed to a phase separator; introducing gas from the phase separator to a nitrogen wash column; introducing liquid nitrogen to the top of the nitrogen wash column; withdrawing ammonia synthesis gas from the top of the nitrogen wash column; withdrawing liquid from the phase separator and expanding the liquid before sending the liquid to a separation column; withdrawing a nitrogen enriched gas stream from the top of the separation column; withdrawing a nitrogen depleted liquid stream rich in methane from the bottom of the separation column; and vaporizing the nitrogen depleted liquid stream rich in methane to form a pure methane product.
 17. The process according to claim 16, wherein the feed gas comprises at least 5% nitrogen.
 18. The process according to claim 16, wherein the separation column contains a bottom reboiler and at least a part of the feed gas and/or a gas derived from the feed gas is used to heat the bottom reboiler.
 19. The process according to claim 16, wherein feed gas is cooled and partially condensed to form a gas and a liquid and at least a part of the gas resulting from the partial condensation is used to reboil the separation column.
 20. The process according to claim 16, wherein liquid from the bottom of the nitrogen wash column is sent to the top of the separation column.
 21. The process according to claim 16, wherein the separation column has a top condenser and wherein liquid nitrogen is vaporised in the top condenser, the nitrogen enriched gas stream is partially condensed in the top condenser and the liquid from the phase separator is sent to an intermediate region of the separation column.
 22. The process according to any of claim 16, wherein the separation column has no top condenser and wherein liquid from the phase separator is sent to the top of the separation column.
 23. The process according to claim 22, wherein liquid from the bottom of the nitrogen wash column is vaporized in the heat exchanger.
 24. The process according to claim 16, wherein the liquid sent to the separation column from the phase separator contains at least 65% methane and at least 5% nitrogen.
 25. The process according to claim 16, wherein the liquid sent to the separation column from the phase separator contains at least 70% methane and at least 5% nitrogen.
 26. The process according to claim 16, wherein the liquid sent to the separation column from the phase separator contains at least 70% methane and at least 10% nitrogen.
 27. An apparatus for the production of ammonia synthesis gas and pure methane by cryogenic separation of a feed gas containing at least 75% hydrogen as well as methane and nitrogen, the apparatus comprising: a heat exchanger configured to cool and partially condense the feed gas; a phase separator in fluid communication with the heat exchanger, the phase separator configured to receive a partially condensed feed from the heat exchange and separate the partially condensed feed into a liquid and a gas; a nitrogen wash column in fluid communication with a gas outlet of the phase separator such that the nitrogen wash column is configured to receive gas from the phase separator; a conduit for sending liquid nitrogen to the top of the nitrogen wash column; a conduit for removing ammonia synthesis gas from the top of the nitrogen wash column; a valve for expanding liquid from the phase separator connected to the separation column via a conduit; a conduit for removing a nitrogen enriched gas stream from the top of the separation column; a conduit for removing nitrogen depleted liquid stream rich in methane from the bottom of the separation column; and a device configured to vaporize the nitrogen depleted liquid stream rich in methane to form a pure methane product.
 28. The apparatus according to claim 27, wherein the separation column has a bottom reboiler and comprising a conduit for sending at least a part of the feed gas and/or a gas derived from the feed gas to heat the bottom reboiler.
 29. The apparatus according to claim 28, further comprising means for cooling and partially condensing the feed gas to form a gas and a liquid and means for sending at least a part of the gas resulting from the partial condensation to the reboiler.
 30. The apparatus according to claim 27, further comprising a conduit for sending liquid from the bottom of the nitrogen wash column to the top of the separation column.
 31. The apparatus according to claim 27, wherein the separation column has a top condenser and comprising a conduit for sending liquid nitrogen to the top condenser, a conduit for sending the nitrogen enriched gas stream to the top condenser and a conduit for sending the liquid from the phase separator to an intermediate region of the separation column.
 32. The apparatus according to claim 27, wherein the separation column has no top condenser and comprising a conduit for sending liquid from the phase separator to the top of the separation column.
 33. The apparatus according to claim 32, further comprising a conduit for sending liquid from the bottom of the nitrogen wash column to be vaporised in the heat exchanger. 